Schmidt Cassegrain
The Schmidt-Cassegrain telescope can provide excellent optical performance for a system with only three optical elements – a primary mirror, a secondary mirror, and a Schmidt corrector – the number of configurations possible is rather large. The corrector, for example can be placed near the focus of the primary mirror or near the center of curvature of this mirror. The first configuration is compact, the second has approximately twice the tube of compact design. Of course, intermediate forms are also possible. The other parameters also permit variety. The focus may occur in front of the primary or behind that mirror. Furthermore, the mirrors may be spherical or have aspheric surfaces. Finally, there are systems, designed to have a flat focal surface for photography and CCD-Imaging, and those with a curved focal surface.
Within the Schmidt-Cassegrain family, the most important distinction, and one that is most closely related to the needs of the amateur, lies between the systems with curved and those with flat focal surface.
Visual Schmidt-Cassegrain systems generally have strongly curved focal surfaces. This stems from a high secondary magnification which permits the secondary mirror to remain relativelly small. Because these systems have been designed primarily for visual use, the designer must attempt to keep the diameter of the secondary mirror less than 30% the primary's diameter. Because of the curved field, these systems are not optimal for wide-field astrophotography unless a field flattener is used to fit the concave focal surface.
Flat-field Schmidt-Cassegrain systems are optimized for wide-field astrophotography on flat film or CCD. The flat focal surface is achived, as it is in Cassegrains, by making the radii of curvature of the primary mirror and secondary mirror equal. As a result of this measure, the diameter of the secondary mirror becomes rather large –45% to 60% the diameter of the primary – when the focal surface lies behind the primary mirror. These photographic systems are poor for visual use at high magnification.
Design and Optimization
Important constants for the Schmidt-Cassegrain are the relative power of the Schmidt corrector and the deformation constants of the primary and secondary mirrors. Before the design procedure is started, it is important to define what demands are to be made with respect to off-axis aberrations. When a sharp axial image is the only requirement (i.e. no correction of coma and astigmatism is necessery), then both mirrors can remain spherical because the corrector lens can completely eliminate spherical aberration.
In the case of two spherical mirrors, coma can be eliminated by moving the corrector farther from the primary mirror. Because the corrector has been moved, the system is no longer a compact design.
To maintain a compact design, for example, we might choose to correct coma by aspherizing one of two mirrors. We have so far not taken correction of astigmatism into consideration. If a compact Schmidt-Cassegrain that is both aplanatic and astigmatic, then it is necessery to aspherize both mirrors.
Applet Tag:
- param name = "Title" value = "Two Mirror Catadioptric: Schmidt Cassegrain Telescope"
- param name = "Corrector Diameter" value = "150.0"
- param name = "Corrector Thickness" value = "5.0"
- param name = "Corrector Location" value = "500.0"
- param name = "Corrector Glass" value = "BK7"
- param name = "Primary Focal Length" value = "500.0"
- param name = "System Focal Length" value = "2000.0"
- param name = "Back Working Distance" value = "200.00"
- param name = "Half Field Angle" value = "0.45"
- param name = "Central Obstruction" value = "0.00"
- param name = "Plot Scale" value = "0.020"
- param name = "TF Start Angle" value = "0.00"
- param name = "TF End Angle" value = "0.45"
- param name = "TF Defocus" value = "0.05"
- param name = "Number of Arms" value = "36"
- param name = "Number of Rings" value = "10"
- param name = "Ray Density" value = "10"
- param name = "Scale Type Index" value = "3"
- param name = "Ray Pattern Index" value = "0"
- param name = "Merrit Function Index" value = "2"
- param name = "Term Index" value = "0"
- param name = "Glass Catalog Index" value = "2"
- param name = "Coma Correction Index" value = "1"
- param name = "Monochr. Color Index" value = "10"
- param name = "WL1 Color Index" value = "1"
- param name = "WL2 Color Index" value = "4"
- param name = "WL3 Color Index" value = "9"
- param name = "Text Color Index" value = "10"